Electrical circuits



C. TRAINS ELECTRICAL CIRCUITS @W53 im? M4K.

Filed July 3l, 1936 G SheeS-Sheet l l nic. Bms

EBECTRICAL CRCUlTS Filed July 31, 1936 6 Sheets-Sheet 2 T0 .lf/R57' DETECTOR' I C. TWVIS Zvl'gr l ELECTRICAL CIRCUITS Filed July 3l, 1936 6 Sheets-Sheet 3 L CONTRO/.LED OSC/LLRTO/ C. TRAVIS EIJECTBICAL CIRCUITS @III 2% IMI 1956 6 Sheets-Sheet 4 Filed July 5l I I I l I I I I I I I I I I I I I I I I I I I ...I

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ELECTRICAL CIRCUITS Filed July 3l, 1936 6 Sheets-Sheet 6 n l l l l I I s I t 11.7 1 i 4, ma E v l l l l -f M 2 I vl I l l l l Y l Y T I x @da y Pz/14.

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Patented Apr.g29, 1941 UNITED STATES `1" .1\Ti:`1\rr oFFlcE annexation. omool'rs' l Charles Travis,

Philadelphia, Pa., asxignorplry mesne assignments, to Philco Radio and Television Corporation, Philadelphia, Pa., a corpo'- ration of Delaware Appao'ouon myvv s1, 193s, soriol'No. 93,13?.

(c1. 25o-2o) 17 Claims.

This invention relates to superhet'erodyne radio receivers employ-ing an automatic frequencycontrolled oscillator by which more exact tuning of the radio receiver may be obtained, the principal object of the invention being to provide an improved automatic frequency-controlled oscillator for use in such a. radio receiver.

,lator in such a way as to minimize the mistuning.

For example, 'if the radio receiver were mistuned by say, k. c., a control voltage might be generated which would shift the oscillator frequency receiver to. be mistuned by only 0.10 k. c., which results in improved reception and also permits the use of automatic and remote control devices and the like, which heretofore have not been sufiiciently precise to tune'accurately and; dependably a' receiver of the superheterodyne type.

The primary object of the present invention 4is to provide a practical automatic frequencycontrolled oscillator suitable for use in multiband superheterodyne receivers and adapted for manufacture by commercial methods.

Another object of the invention is to provide an improved controllable oscillator employing a vacuum tube as a controllable impedance, and

, to provide "means whereby this impedance may appear as a. pure reactance from which the resistive component inherently present in vacuum Itube circuits has been eliminated.

Another object of the invention is'to provide a controllable oscillator-'employing a differential circuit including a plurality of vacuum tubes,

' Which/tubes operate as pure reactances and from which the .resistive component has been eliminated.

'Another object of the invention is to provide a controllable oscillator employing a vacuum tube circuit in which vacuum tubes are utilized as impedances and the impedance presented by the tubes is determinedby the mutual conductance of the tubes, .and to provide means for minimizing the change in effective impedance due to "ariations in the mutual conductance which might be caused by variations in the cathode temperature, anode voltage, aging ofv the tubes, and similar circumstances.

Still another object of the invention is to prolvide an automatic frequency-controlled oscillator such a direction as to cause the adapted to cause variation of the frequency with T respect to the control voltage in a symmetrical manner about each side ofthe mean ,'frequency.

Still another object of the invention is to provide an automatic frequency-controlled oscillator so arranged that frequency modulation due to hum pick-up andV other extraneous signalswhiich may be induced in the leads of the circuit, is minimized.

Another objoot of the invention is to'provide a resonant control reactancefor use in supplying a quadrature voltage to the'control tubes in such a Way that substantially uniform control sensitivity may be obtainedv over the frequency band within which the oscillator frequency may be established.

Other 4,objects and features of thel invention will be apparent from the following description and the accompanying drawings. 1

in the drawings:

Fig. 1 is a schematic block diagram of a super,- heterodyne receiver employing an automatic frequency-controlled oscillator such as that provided loy my invention and showing the use of such oscillator in conjunction with an automatic or remotetuning device;

Fig. 2 is a diagrammatic illustration of a preierred form of the discriminator'unit employed I trating the phase-correction ofthe control tube Vused. in. cooperation with van electron coupled oscillator;

Fig.. 4 is a similarv illustration of another form of the oscillator unit wherein the quadrature voltagey necessary for. control is obtained from the oscillator grid circuit rather than from the output circuit of the electron coupled oscillator;

Fig. 5 is a diagrammatic illustration of a preferred form f of 'the' automatic frequency-controlled oscillator; employing a differential control circuit;

Fig. 6 is a similar illustration of anlalternative form of the same type of oscillator unit;

Figs. 7 and 8 are diagrammatic illustrations of modifications of the automatic frequencycontrolled oscillator containing `certain rene- Y ments in the control circuit;

Figs. 9 and 10 illustrate certain operating characteristi-cs of the control tubes of Figs. 3 to 8;

Fig. 11 is a graph showing the several types of operating characteristics which maybe ob- I tained with several different control reactances; and

Figs. 12, 13 and 14 show the various different types of control reactances, those of Figs. 13 and 14 providing uniform control sensitivity in accordance with my invention.

The general operation of a superheterodyne radio receiver employing an automatic frequencycontrolled oscillator may be understood by reference to the system of Fig. 1, wherein the incoming signal is amplified in a radio frequency aznplifier,.then supplied to a first detector in which it is heterodyned with a locally generated signal whose frequency is controlled, the heterodyne or intermediate frequency signal being then amplified in the subsequent I. F. amplifier.' The I. F. signal is then demodulated in a second detector, and the demodulated or audio frequency signal is Vamplified andv supplied to some utilization means, such as the conventional loud speaker L. S. The automatic control of the oscillator frequency is obtained by the use of two units, the first unit being a discriminator winch forms a controlor A. F. C. (automatic frequency control) signal, indicating the extent and the direction of the deviation of the intermediate frequency signal from the frequency to which the discriminator is turned, and the second unit comprising a. controllable reactance for the frequency determining circuit of the oscillator whereby its frequency may be controlled over a certain range by this control or A. F. C. signal. As indicated in Fig. 1, a portion of the I. F. signal energy is supplied to the discriminator which supplies the A. F. C. signal to the frequency-controlled oscillator to control the locally generated signal applied to the first detector. The discriminator may also supply an A. V. C. (automatic volume control) signal to the parts of the system, as indicated. As shown in Fig. l, the frequency-controlled oscillator may be employed in conjunction with an automatic or remote tuning device, as will be described hereinafter.

A suitable discriminator unit for use in connection with the present invention is shown indetail in Fig. 2.. As noted above, I. F. signal energy preferably obtained from the output of the I. F. amplifier' is supplied to the discriminator. 'I'his modulated intermediate frequency signal may be supplied to the control grid' of the amplifier tube V1 (Fig. .2) which tube' may have a parallel resonant circuit I as its principal output load. Loosely inductively coupled to the resonant circuit I is acenter-tapped resonant circuit 2, which is connected through diodes D1 and D2 to serially-connected loading-resistances R1 and R2 and to ground through blocking condensers Ctr and Cba. These condensers are shunted by resistances Rs and R4, respectively, whose common connection may be connected to ground. The center tap ofthe inductance L of the resonant circuit 2 is connected to the common connection of the resistances R1 and R2, and is also connected to the anode of the amplier tube V1 through a blocking condenser Cba. The anodes of the two diodes D1 and D2 are grounded for intermediate frequency signals by means of the condensers C111 and Cba.

The signals applied to the cathodes of the two diodes will include the signal across the output of tuble V1, which signal ground and the center tap of the inductance L across lthe resistances R1 and R2. For signals of the frequencies involved, the resistances R1. and

will build up between R2 are effectively in shunt with each other. The 75 v magnitudes of these A. C. voltage induced in the inductance Lwlll add vectorially to the A. C. voltage built up across the resistances R1 and 2, and the phase relation between these two A. C` voltages will depend upon the frequency of the incoming signal. If the circuits I and 2 are tuned to the intermediate frequency, for signals of that frequency the A. C. voltage inducted in L will be out of phase with respect to the A. C. voltage across the shunt resistances R1 and R2. However, for frequencies other'than the intermediate frequency, the vector relation between the A. C. voltage induced in L and the A.v C. voltage across the resistances will ,l

diifer by a phase angle greater than 90 or less than 90, depending upon whether the frequency is higher or lower than the intermediate frequency. Thus, the scalar magnitude of the A. C. voltage between the anodes and cathodes of the two diodes D1 and D2 will depend upon-the frequency of the incoming signal. If, Afor example, this signal were of lower frequency than that of the resonant frequency of the tuned circuits I and 2, the scalar magnitude of the signal applied tothe diode D1 might be greater than that of the signal applied to the diode D2. On the other hand, if the incoming signal were of higher frequency than the said resonant frequency, the scalar signal magnitudes would be reversed.

Due to the rectifying action of the diodes D1 and Dz, a direct current will now from the anode to the cathode of diode D1, through inductance L and back to the diode anode through the resistance R1. The voltage applied to diode D2 Will likewise cause a direct current to flow from its anode to its cathode, through the inductance L and back to the diode anode through the resistance R2. Thus, a D C. across each of the resistances R1 and R2. and the potentials will depend upon the scalar magnitudes of the voltages applied between the cathodes and anodes of the diodes Di and D2. Where the incoming signal frequency is the same as the resonant frequency of the two tuned circuits I and 2, the voltage across R1 will be equal in amplitude to that across R2. and consequently, no current will flow through the series resistance RaR4 since the two ends of this resistance will be at the same potential. If, however, the incoming signal is of lower frequency than the resonant frequency of the tuned circuits I and 2, the potential across R1 might be greater inamplitude than that across Rz, causing a current to ow through R: and R4 and thus making the end of Ra negative with respect to the end of R1. If the common connection of these resistances is grounded, the potentials-thereacross will be balanced with respect to ground, that is, as the one terminal becomes positive with respect to ground, the other will become negative by an equal amount. These rectiied signal voltages may be filtered by the resistance-capacitance filters 3 and 4, so that across the output of the discriminator unit, there are obtained two control signal voltages balanced with respect to ground, the amplitudes and polarity of which will be determined by the departure ofthe LF. sig- Vnal frequency from the resonant ,frequency of the A. F. C. signals with respect .to ground will be As will be explained built up across Rs and R1.

potential will be built up from R3, the end o'fRJ. connected to the diode Dn may bel grounded. Where two A. F. C. signals are employed, as in the differential circuit described'hereinafter, then the junction of R3 and R4 should be gro'unded in order to obtain equal and opposite signals.

The particular diode connections shown in the discriminator of Fig. 2 oiers certain advantages which are not obtained unless the cathodes are connected to the inductancevL and the common connection of the resistances R3 and R4 isk grounded. When these connections are used, the circuit is so arranged that leakage current, paths through the cathode-heater connections to ground will not cause a change in the differential A. F. C. bias.

Suitable connections to energize the heaters from the filament winding of the power transformer are shown in Fig. 2. The same winding may be used to energize some or all of the other` tubes in the receiver. However, to prevent large potential differences from building up between the cathode and heater winding and to minimize' hum the center-tap of the nlament winding should be grounded. If leakage current should flow `between the heater and cathode the effect of such leakage in either diode is to place a small unidirectional voltage between the common connection of. R1Rz and ground. This will cause equal currents to flow to ground through R1Rs and RzR4, but will not cause a difference in potential across RaRra'nd thus no A. F. C. bias when the differential circuit is used. This is not the case when the diodes are used in the reverse direction, when leakage in either diode will produce an extraneous potential in the discriminator circuit.

Since the anode,A of tube V1 constitutes the point in the receiver at which the intermediate frequency signal is of greatest amplitude, an automatic volume control voltage may lconveniently be obtained at this point by rectifying a portion of the output signal by means` of the diode Da, whose cathode may be connected to the cathode of tube V1 and whose anode may be connected to the anode of tube V1 ,by means of a condenser Cbr. Since the I. F. signal amplitude at thislpoint is very high, the diode anode may be biased back by a large negative delay voltage obtained from source 5 and thus a highly eilicient source of automatic volume control voltage is obtained. This voltage may be filtered `by a resistance-capacitance filter 6 and may be applied to the several preceding ampliiler units to control the gain thereof, as indicated in Fig. l.

In Fig. 3, there is illustrated one form ofthe frequency-controlled oscillator unit which may be employed in connection with the discriminator unit of Fig. 2. This oscillator unit comprises an oscillator tube V2, an oscillator tank circuit 1 'including a variable condenser C1 and an inductance L1 in shunt therewith, and a control tube Va. 'I'he signal across the tank circuit is supplied to the control grid of the electron-coupled oscillator Va whose anode is connected back to the '3 ti'ckl'er coil Le to provide the necessary feed-back.

The cathode of the oscillator may be connected to ground through the self-biasing resistor-condenser combination 8 and the control grid may be biased by connecting it to ground through resistor 8. The outputl or oscillator signal which is supplied to the first detector may be obtained from the control grid, as shown. A signal voltage of the oscillator frequency .but in phase quadrature with respect to the oscillatory input signal voltage is obtained by means of the' control reactance C, as described more fully later, and is applied via connection I0 to the grid of the control tube Va, thus causing the control 4tube to produce an output signal current in phase quadrature with respect to the input signal voltage of the oscillator, which, of course, is in phase with respect to the signal voltage between the anode and cathode of the control tube. Thus, the output current and output voltage of the control tube are in phase quadrature with respect to each other, and their relative magnitudes will depend` upon the mutual conductance of the tube and the magnitude of the input signal. Therefore, neglecting for the moment the effect of the inphase signal component discussed hereinafter, the control tube will appear as a reactancewhose magnitude is determined in part by the mutual conductance of the tube, which in turn is determined by the bias'applied to the control grid of the tube, the mutual conductance being greater y when the grid is less negative. ence, the magnitude of the reactancepresented by the control tube will be determined by the bias upon the grid of the tube, and by applying the A. F. C. voltage obtainedfrom the discriminator as a bias, the tube may be utilized to control the resonant frequency of the oscillator tank circuit in such manner that if the intermediate frequency signal of the receiver is less than the resonant frequency of the tuned vcircuits i and 2, the resonant frequency of the oscillator is increased, to thus increase the heterodyne signal frequency and to bring the receiver more nearly into tune. It will be noted that .fthe degree of control is determined by the degree of mistuning and thus the type of control action here involved is that known as a. servo" action. By utilizing a sensitive control, the receiver can bebrought to very nearly perfect tuning, but never to exactly perfect tuning. The slight misalignment, however, is not sutilcient to impair the operation of the receiver.v

It will be seen :that the device of Fig. 3 requires the application thereto of only one A. F. C.

bias ,voltage which may be obtained from the device of Fig. 2 as above indicated.

, In the adaptation of such an automatic frequency-controlled 'oscillator to a practical receiver, it was found that this control tube presented an appreciable undesirable resistance to the tank circuit, which resistance, if it did not overload the oscillator until the latter ceased to function, varied with both frequency and A. F. C. bias thus causing undesirable changes in the amplitude of signal supplied to the iirst' detector.`

This undesired resistance is due principally to three sources described hereinafter.

the practice of the invention, this resistance may be either balanced out identicallyy or modified to a value independent of both frequency and A. F. C. bias and such as not to affect the oscillator performance adversely, by introducing a direct-phase signal component in the control tube input circuit, which component causes the control Following tube to vappear as a resistance of opposing polarity as compared with the undesired resistance.

The three general sources of undesired resistvvtau, it was round first that due to the inherent capacity between the control grid and plate of the control tube V3, indicated by the dotted condenser Cgp in Fig. 3, the control tube tends to present an appreciable resistance to thetank circuit. This maybe seen from a consideration of the lvoltage relations in the input circuit of the control tube. As will be shown later, the control reactance,

which comprises the condenser C of Fig. 3, shouldv preferably be a capacitive reactance, although not y necessarily a condenser. Therefore, the control negative resistance.

grid'of the control tube V3 is capacitvely connected to ground by means of the control reactance, and is capacitively coupled tothe anode by means of the inherent grid-to-anode capacity of the control tube and the external wiring, Consequently, a change in anode voltage will cause a proportional change in voltage of the control grid, which in turn will cause a variation in the space current of the tube. Thus, this coupling in troduces an undesired in-phase input signal. As an increase in a'node voltage will cause an increaseV in anode current, the tube acts as a resistance and Vwill .absorb energy from the tank circuit. If, on

the other hand, the control reactance were inductive, the resistance of the control tube would be negative and it would cause'the control tube to oscillate, but at a frequency other than thatdesired. In fact, inv general, the frequency may shift /back and forth between that determined by the oscillator proper and that at which the control tube tends-to oscillate, and the result is a. signal generally useless for the present purpose.

A second source of resistance is the actual anode-to-cathode eiective A. C. resistance of the control tube itself. Thus, independently of the grid potential, if the anode voltage is varied, the anode current will vary in such a way as to cause absorption of energy by the control tube. In vacuum tubes of the type generally suited for this purpose, the amplicationfactor'is substantially constant and thus this second resistance will vary inversely with the mutualconductance of the tube, i. e. with A. F. C. bias.'

The third source of resistance is due to an undesired in-phase signal component in the signal obtained from the control reactance and supplied to the input circuit of the control tube. While the cause of this in-phase component is not4 completely understood, it may be said that in part at leastit is due tothe oscillator grid leakl In part it may be due to capacitive coupling or coupling' of other nature in the oscillator itself. Still another source might be the resistance through which energy may belsupplied to the output anode be considered in terms of 'the equivalent directphase input signal whichwould cause such a resistance if the tube were replaced by an ideal amplifier. For practical purposes it matters not whether such equivalent signal is actually present in the circuit or is simply assumed to bethere. In either event the deleterious elect of the undesired resistance may be eliminated by supplying to the input circuit a direct phase signal of opposing phase relationwith respect to the phase of the undesired equivalent signal. Thus, brieily,

' an ideal amplifier supplied with an input signal in phase quadrature relation with its output signal is desired to form a pure reactance. Due to physical limitations of available apparatus, however, the impedance of the control tube will con- Atain an undesired resistive component due physiof the oscillator.y In arlyV event an undesired dil rect phase signal component may be present.

-As described abovefan input signal in phase with the signal in the control tubecutput will cause the tube to appear as a positive resistance lbe uniform over and absorb energy while an input signal in phase opposition will cause the tube to appear as a the invention, the undesired eiective resistance component of the control tube output circuit may Following the practice of Y cally at least in part, and hypothetically entirely, to an undesired direct phase signal component in the input circuit. By the invention, a phase-correcting direct phase input signal is provided which opposes the undesired signal component, ilsahust eliminating the deleterious effects'caused In general, the equivalent undesired signal will be in-phase with the control tube output circuit voltage. It is likewise independent of A. F. C. bias and will vary with frequency only if the effective capacitance of the control reactance varies with frequency. Assuming, however, that the undesired signal is independent of both frequency and A. F C. bias, the effect of the resistive component in the control tube output onthe resonating circuit will vary with both of these factors. A suitable direct phase voltage in phase opposition to the undesired signal may be obtained, following the practice of the invention, by connecting an adjustable condenser C2 of proper capacity between the control grid of .the control tube and the anode end of the tickler coil La. As the voltage at the anode end of the tickler coil Le is out of phase with that supplied to the anode of the vcontrol tube Va, and asl this point is capacitively coupled to the grid by the condenser C2, it will supply an input signal in-phase with vthe voltage across the tickler coil La and thus 180 out of Aphase with the plate voltage ofthe control tube V; and 180 out of phase with the undesired input voltage and dependent upon the control reactance in the same way. By properly proportioning the capacity of C2, the voltage providedv by the phase-correcting ycircuit may be made equal in magnitude to the undesired signal andas it is opposite in phase, the two will oppose -each other, leaving only the quadrature component.

Preferably the undesired in-phase'signal component is opposed by an equal and opposite signal obtained as above described. In certain instances, however, an alternative method of ob- -Thus, the undesired signal due -*to the interelectrode capacity and the control reactance will the band and proportionaly to the control tube grld-to-plate inter-/electrode capacity divided by the capicitance C. The equivalent undesired direct phase signal due to the other two sources may likewise in. many instances be uniform over the band. Now the con trol reactance circuit is substantially a constant vcurrent circuit and thusrthe voltage due to the series resistance will be substantially a constant over the band, and as it will be in phase opposition to the undesired signal in the input circuit of the control tube due to the 180 phase shift between input and output signals of V2, this signal due to the resistive component of the control reactance is a proper signal to provide the desired phase correction. It Will be notedA that lthe resistance r is adjustable and that the resistance supplying energy to the output anode of V2 is suiiiciently large to be negligible compared with the low impedance control reactance in shunt with it.

While two means of obtaining .the voltage m phase opposition to the undesired in-phase sigi nal have been shown in Fig, 3, it will be understood that either or both may be employed dependinguponthe particular circumstances. In either case, when the control tube is phase-corrected in accordance with my invention, the.

voltage between its anode and ground is in true phase quadrature with the voltage supplied by the control reactance to the grid.

It will be noted that in Fig. 3 the signal supplied to the control tube is obtained from the output circuit of an electron coupled oscillator. As is known, in tubes of this type, the space current to the output anode is determined largely by the voltage of the oscillator grid and is sub stantially independent of the voltage of the oscillator anode. Thus, by providing a reactive output circuit or control reactance, tthe desired quadrature voltage may be obtained thereacross, as described more fully hereinafter, and supplied to the control grid of the control tube. The gain' of the control tube and thus its effective reactance may be controlled by the A. F. C. 'bias as indicated.

In Fig. 4 there is shown an alternative method by which a voltage in quadrature phase relation withv respect to the oscillator grid voltage may be obtained. In this case, the oscillator grid is connected to the control reactance C' by a large resistance R. and an inductance L. Preferably the inductance L' and control reactance C' should be tuned to be resonant in the middle oi' the band over which the oscillator may be tuned, and the resistance of R should be considerably greater than the impedance of C or L'. Under these conditions, the signal across the control reactance C 'will be exactly in phase quadrature with that across the output circuit of the control tube Vs for the resonant frequency of L'C' -and substantially so over the rest of the band. Thus the signal across the control reactance C constitutes a proper signal to be applied to :the control grid of the control tube. It will be noted, however, that this voltage is 180 out of phase with respect to the equivalent voltage obtained in Fig. 3 and hence the polarity of the A. F. C bias should be reversed to obtain the same direction of control in the two cases. In other words, in Fig. 3 an increase in A. F. C. bias will cause a change of oscillator frequency in the same direction as that caused 'by a decrease in A.F. C. bias in the circuit of Fig. 4. The discriminator of Fig. 2 provides either signal. The

phase correction may, however, be obtained in exactly the same manner in the two cases, that is by the use of the variable condenser C2. Where' the anode of V3 may be connected to the anodev end of In instead of the grid end of L1.

In Fig. 5, there is illustrated a preferred form of the frequency-controlled oscillator unit which is preferably employed with the discriminator unit of Fig. 2. In this instance; the cathode of the oscillator V2 is connected to ground through a self-biasing resistance-capacitance illter and the control grid is biased by tapping in on the resistance of this filter. This preferred oscilla- .tor unit employs a phase-corrected control circuit which has certain important advantages, as will be pointed out more particularly hereinafter. In this instance, two control triodes in the form of a double triode ltube V4 are used and are so coupled into the tank circuit that the eiIect of one is in the opposite direction to the effect 'of the other. Thus, for example, an increase in bias on one triode might 'cause an increase in oscillator frequency, whereas an increase in 'bias on the other triode would cause a decrease in oscillator frequency. Therefore, in this instance two A. F. C. control voltages are used, one of which increases when the other decreases, and

shown in Fig. 2, the desired control voltages may be obtained..

The circuit of Fig. 5 is generally similar to that of Fig. 3 insofar as the method of obtaining the quadrature voltage is concerned. It will be noted, however, that the same quadrature signal is applied to each control grid of the triodes of tube V4. In this diierential circuit, the grid-toplate inter-electrode capacitance of each triode acts as the phase-correcting capacitance for the other triode and thus the use of a phase correcting resistive component in the control reactance is unnecessary. The control reactance C is connected between the grids and ground as before. If the respective anode inductances of the two triodes are equal and if the respective inter- `electrode capacitances are equal, as would be the will appear as pure reactances due to the quadrature voltage applied to their respective grids. In case the inter-electrode capacities of the triodes should diifer for some reason, the lesser capacity can be increased by the use of a small phasing condenser analogous to the use of such a condenser shown in Fig. 3.

The deleterious eect of the resistive component of the control tube reactance which is due to the second source, that is to the anode-to-cathode resistance of the control tubes, need not, in general, be opposed where two control tubes are used since under these conditions this resistive component does not vary with frequency or with A. F. C. bias 'and thus may be overcome by increasing the feed back from the oscillator. It will be noted that the resistance component of each tube will be in shunt with each other and withthe resonating circuit. Each resistance will vary with A. F. C. bias but not with frequency. However, as the A. F. C. bias is varied it will cause the resistance of one tube to increase, while' that of the other decreases. Thus the two variations oppose each to the input signal voltage ftriode an the other triode.

' than the electrical circuit shown in Atwo diodes D4 and D5 and other and except for` second .ordervefiects the' resultant resistive component due to the two will be unchanged.

. In the operation of this device; a signal voltage of the oscillator frequency, but in phase quadrature with the oscillator input signal voltage,V is applied to the grids of the two triodes .of tube V4. thus causing each triode to produce an output I anodes fof the diodes signal current lin phase quadrature with respect of the oscillator. which, of course, is in phase with respect to the signal 'voltage -between the anode and cathode of one 180 outof phase with respect to the signal voltage between 'I'hus it will be seen that the the anode and cathode of v output current and output voltage of each triode are'in phase quadrature with respect 'to each othenandthe magnitudes of these signals will depend upon the mutual conductance of the triodes and the magnitude of the input signal. Therefore. each triode will appear as a reactance (one positive and one negativelwhose magnitude Y is determined in part by the mutual conductance of the triode. The mutual 'conductance of the triode, however, is determined by the bias applied to its control grid. being greater when the'- grid is less negative and. therefore. the magnitude of the odes of the diodes are F. C. leads. The second grid is connected to the diode anodes.

are connected together and the resistance Rv. 'I'he cathconnected to the two A.

to ground through When one of the A. F. C. leads is negative with respect to ground. current will flow through. the diode connected yto that lead and thence tc-ground. causing negative bias tov build up acrossRa proportional to the negative A. F. C. bias. No current will ow through the other diode as its cathode is positive with respect to its anode. Thus the bias supplied to the second grid of Va will be proportional to the voltage of whichever A. F. C. lead is negative. This bias when applied to Vs will decrease the frequency control sensitivity of the circuit when the receiver is badly mistuned but will not affect the operation whenl the receiver has' been approximately tuned.

reactance presented by the triodes will be determined by the bias upon their grids, and by applying the A. F; C.voltage as a bias, the4 triodes may be utilized to controlthe resonant frequency of the tankvcircuit in such a way that if the inter mediate frequency signal of the receiver were less resonant frequency of the tuned circuits I and 2, then the resonant frequency of the oscillatorwould be increased, which would tendto increase the heterodyne signal and thus bring the receiver more nearly into tune, and vice versa.

In Fig. are used and are connected in the manner of the triodes of Fig. 5. The oscillator proper isshown as comprising two tubes V1 and Va, one a triode and the other a multi-element tube ofthe electron vmixer type. By connecting the input grids of these two'tubes together and using only the anode of the triode as the oscillator anode. a more powerful oscillator may be obtained and the second control grid of the 6, two separate control tubes Vs and Va.

electron coupled oscillator- Thus it is possible to tune the receiver manually without having the A. F. C. circuit lock in on a strong station and dragl this station through many channelsl before releasing it. The receiver lis thus enabled to receive some other station.

The "cathodes of the two control tubes are connected to ground through'blocking condensers Cbt` and'Cbc and through a self-biasing network so arranged that the D. C. potential of the cathode of tube -Vs may be slightly modified with respect to that of tube Ve by variation of the point :at which the potentiometer P is grounded. In this manner, slight variations in the mutual conduct-- ance.; of the two tubes may be equalized. It is, of course, desirable that the two tubes have identical electrical characteristics.

Aside from the above noted differences, the device of Fig. 6 is similarto that of Fig. 5

and operates in the same manner.

In Figs. -7 and 8 are shown two further refinements which while not essential to the attainment of commercially satisfactory results, may

. xbe used 'to advantage in certain instances. In

is left freefor other purposes. such as described below. It will be understood that other types of oscillators may be used, such as those shown in the other illustrations. The quadrature signal may be obtained across a control reactance C connected to the output of Vs.

' Suitable Icontrol signals for this purpose may be obtained in several different ways. In some cases, it is desirable that the A. F. I. circuit of the receiver be inoperative while the receiver is being tuned but operative after the tuning operation is effected. This may be accomplished by providing -a switch of the push-button type or an inertia operatedby movement of the switch mechanically tuning condenser shaft so that when the set is being tuned the switch supplies a negative bias to the second control grid of the tube Va, thus biasing out the quadrature signal, while, lwhen the pushbutton is released or when the movement of the tuning condenser shaft is completed, the switch opens returning the control grid bias to normal and thus rendering the A. F. C. circuit operative.

Similar useful results may be obtained by the Fig. 6. In this circuit. a resistance R1 are used to supply the bias for the second grid of Vs. The

other instead vexpedient may cuits. A signal Fig. 7 a triodel Vn 1s resonant circuit 1 tions. In'this used in cooperation with the to form a source. of oscillabe used in any of the above'cirmay be obtained from the output circuit of Vn and applied to a gain control tube Vn which may contain the control reactance X1 in its output circuit. While shown as a condenser any of the control reactances described hereinaftermay be used here. A quadrature signal is obtained from the control reactance and applied tothe control tube V10. The phase correcting signal may be obtained by means of the adjustable condenser C2 from the inductance L1. Ii desired C: might be connected resistance in the control reactance circuit may be used. Both of these methods have been described in connection4 with Fig. 3. It will be noted that in Fig. 7 the controllable reactance is the output of control tube Vio while the magnitude ofthe reactance is controlled in the gain control tube Vn. In thespeclilcation and claims, by control tube, is meant that tube 'which is connected to supply signal to the frequencydeterminlng circuit of the oscillator and thus forms the control reactance. In the other ngures the control tube has also been used to act as the gain control but this function may be accomplished in a separate tube such as Vn in Fig.`7, which tends to minimize further any variations in generated signal amplitude with A. F. C. bias.

gureLi I c L4 La may be separate inductances closely inductively coupled to each two inductances as before. This to L s or an' adjustable i In Fig. 8 is shown a differential circuit in which the input circuits are supplied with separate quadrature signals in phase-opposition while the output circuits are in vshunt with each other.

The oscillator part of the circuit is similar to that of Fig. 5. It will be noted, however, that the anodes of control tubes Viz and Vn are connected together and to L1 of resonant circuit 1. The control reactance comprises theinduct'ance Lp, condenser C and resistance c'. The inductances Le and Lv may be equal and closely coupled and thus the signal across In will be equal and opposite\that across Le. The impedanceof Le may beconsiderably greater than that of C, in which case the reactance of the unit will be substantially that of, the condenser. quadrature signal will develop across La and an equal and opposite signal across Lv.. These two out-of-phase signals are applied to the grids of Viz and V13.V Equal and opposite A. F. C. biases may. likewise be applied to these grids and the differential reactance obtained in the `combined output circuits As before the undesired directphase signal due to the inter-electrode capacity of the one control tube .opposes that of the other.

While the undesired resistance due to the anode-to-cathode resistance of the control tube and due to the direct phase component of the quadrature signal will to a certain extent remain constant independent of A. F. C. bias as before,"

this component may be opposed by an adjustable resistance r in the control reactance. This resistance may be small and there will lbe builtl up across it a small direct phase component which is applied in like phase to the grids of each tube. In the circuit as shown, an increase in output voltage of V12 and Via will cause'a decrease in grid voltage of each tube and thus r may be used to introduce a signal` to oppose an effective energy absorbing resistance, in the resonant circuit. VBy connecting the anodes of by the amountindicated so that the zero-controlled oscillator `frequency for any particular manual adjustment of the tank circuit would correspond with Fn. change in the A. F. C bias about the zero point indicated by the dotted` line would cause the oscillator frequency to shift about the frequency F0. If. however, the mutual conductance of the tube were to change for any reason, for example, due to a change in the plate voltage or the cathode temperature, or due to aging ofthe tube itself, the mutual conductance curve will change, say to a position shown `by the broken-line. Consequently, the mean frequency about which the oscillator frequency is varied will shift to a position F1. However, the remaining elements of the receiver including the tank circuit,.R. F.

circuits, etc., will have been adjusted so that teristic is shown in Fig. 10. In this circuit, one

control tube is adjusted to cause aY change of frequency in one direction, while the other control tubev is arranged to cause a frequency change in the control tubes to La lnsteadof Li a phase cor recting signal of opposite polarity is obtained. Thus by the circuit of Fig. 8 the undesired resistive component of the impedance of the control tubes may be eliminated completely as compared with the circuits of Figs. 5 and 6, in which its effectis reduced to a 4small constant value.

'Ihe use of a differential circuit, such as shown in Figs. 5, 6 and 8, offers certain other impor" tant advantages. This may be seen from a con- (sideration of Figs. 9 and 10. Fig: 9 depicts the action of an oscillator unit employing one control tube, such as shown in Figs 3, 4 and 7, while Fig. 10 depicts the action of an oscillator unit employing differential" tubes, such as shown in Figs. 5, 6 and 8. In Fig 9, there is shown a curve in which the ordinate represents the departure of the oscillator frequency from the frequency which the oscillator would have inthe absence of the control tube. It will be seen that this frequency deviation is proportional to the mutual conductance of the tube and thus, by represent-A ing the abscissa as the grid bias and drawing a conventional mutual conductance characteristic for the control tube, a curve is obtained in which the frequencydeparture is plotted against the A.I F. C. bias In Fig 9, the natural frequency of the oscillator tank circuit may be represented by Fal Due to the addition of the control tube, however, the mean vfrequency about which the oscillator frequency is deviated should be about half the total possible frequency deviation and this is represented by the point F0. the control tube would then be normally biased 'Ihe grid of the opposite direction. Therefore, the change in tuning dueto one tube may be indicated by the characteristicA similar to that of Fig. 9,-whi1e the change in tuning due to the other tube is represented by a curve B sloping in the opposite direction, since the A. F.'C. biases supplied to thetwo tubes are equal and opposite, and thus when the grid of one tube becomes positive, that of the other tube becomes negative with respect tothe normal bias. The actual change in frequency will be determined by the difference between the two and this is indicated by the resultant curve D. The coordinates of the graph in Fig. l0 are the same as vthose of Fig. 9. It will be observed that in this 'instance the actual frequency of the oscillator for one'value of bias will-be the same as that of the tank circuit in the absence of the control tubes, and this value of bias, which will occur when the two tubes are biased at the same point irrespective of the potential of that point, is selected as the mean frequency about which the oscillator is to vary. If now, the mutual conductance of each tube changes by the same amount, then there will be no change whatsoever in the mean frequency. Due to the differential nature oi' the circuit, variations in the mutual conductance of the control tubes which do occur, will be substantially the same for each tube. For example, since the filaments of the tubes may be connected together and since the same plate voltage is applied to each tube,

changes in these parameters would cause substantially identical changes in the mutual conlductance of the tubes. Furthermore. it is advantageous to use for the control tubes a tube havingtwo triode sections in the same envelope, which provides that the two triodes will age at the same rate. It will further be apparent that any hum picked up by the plate leads or grid leads of the tubes will cause equal changes 5 in the equivalent mutual conductance of the Thena positive or negative tubes, and, therefore, the differential circuit is of the'electron-coupled oscillator. This inductfree of frequency modulation due to these ance is inserted in shunt with the inductance sources, Furthermore, the automatic frequency Y of the resonant circuit. However, in tuning the control feature may be removed by simply shortresonant circuit, the capacitance is generally circuiting the A. F. C. Voltage, for example, by varied, and, therefore, the sensitivity of the conmeansof thel switch S shown in Fig. 2, withtrol unit will be proportional to the'ratioeof the out causing any misalignment of the receiver. inserted equivalent inductance and the natural 'I'he normal bias for the two control tubes may inductanoe of the ltank circuit. The sensitivity conveniently be obtained by means yof a com-. therefore, will be proportional tothe frequency, mon resistance in the cathode circuits of the since .a given percentage change'in inductanoe control tubes. This resistance may be conwill cause a proportional percentage change'in nected to ground and serves to establish a norfrequency, regardless of the resonant frequency mal negative bias common to both control tubes. of .the circuit. This is shown in Fig. 11,- which It will be noted that the center point of the`A is a. graph in which the ordinate is control `3 resistances Rs and R4 across which the A. F. C. 15 sensitivity in terms of k. c. frequency shift per l voltages are formed is likely grounded and thus volt of A. I i. C. bias, and the abscissa represents` i the A. F. C. voltages will bias the grids of the the frequency to which the tank circuit is mani control tubes positive or negative with respect ually tuned. The control sensitivity, which in i -to -this normal bias. Due to the diiferential nathis instance is proportional to the oscillator ture of the circuit, the normal bias point will frequency, 1S represented by the curve I. be substantially independent of the A. F. C. If, however, the control reactance were an inbiases. On the other hand, were a single tube ductance, such as Ashown at Xin Fig. 12, then the used, it would be necessary to provide some` rreactance presented by the control tube would be external source of anormal bias or alternatively a negative capacity which would effectively be in operate the control tube self-biased and increase shunt with the physical condenser in the tank the sensitivity of the discriminator unit to overcircuit. Howeve since the frequency of the tank r come the variation in self-bias with variations circuit is manually adjusted by varying the cain A. F. C. bias. pacity of the tuning condenser, and since this It will further be noted that due to the difchange is proportional to the square of the freferential feature of the control, the control senquency, the control sensitivity will then vary as sitivity is made substantially uniform or linear the cube of the frequency and will be in the oppo-l about the mean frequency. This then, provides` site direction from that previously noted. This s. convenient way of obtaining frequency moducondition is shown by curve II in Fig. v11. lation. In other words, while in the present in- It is desirable, however, that the control seny stance, it is desired that the osciiiator should sitivity be substantially uniform over the fre-- f be completely unmodulated, and provide a suby queney band in which the oscillator may be stantiaily pure sinusoidal signal, in the event tuned. which band may extend over a frequency i that it was desirable to frequency-modulate the range of about 2 to 1- To achieve this result, a oscillator for use in' some other circuit, this, capacitive reactance which reflects back as an incould be accomplished `conveniently byl supplying 40 duCtanCe dS Preferably employed. To make the the modulating signal to the control grids of controluniform, the effective -inductance should the two control tubes by an input circuit bal- Vary inversely With frequency and thus a Capacianced -to ground, and linear frequency modulativereactance is required which is inversely pro--V tion would be obtained. It will further be ap- DOItlOnal t0 frequency Squared. As the quadraparent that any degenerative effects due to the'45 ture` voltage supplied to the control tube is proself-biasing resistor and filter in the cathode DOltOnal t0 the COIltIOl reatan, the reatance circuit 0f the two triodes will be balamed out desired Should have the Characteristic that the, by Ithe circuit of the invention, and consequently, product of the voltage therefcross multiplied by variations in the cathode voltage `with respect the frequency Squared iS Substantially constant .to groundwill not cause variations in the freover the band. This condition obtains when a,

quency of the output 'sig-nal.. Additionally, 11; resonant control reactance is used. will be observed that as the' tubes age and the In Flg- 13,` for example, there is shown a conmutua] conductance decreases, the only change tIOl IEaClale COmpIlSlIlg a Sel'leetllned Circuit l in the operation of the circuit will be a slight X1 Which Circuit l5 resonant above the frequency change in its sensitivity as shown by the broken- 55 band of the oscillator. The circuit will then aplines inv Fig., 10. Y pear as a capacitance, but the control reactance contrasted withthe above advantages of the when transformed bythe control tube will appear differential cgntrpl 1l-Cuit, the Com-,rol circuit asanegative inductance which, over the oscillator employing a single control tube, as shown in frequency range. Will Vary practically inversely Figs. 3 and 4 hasfthe advantage that it requires co With frequency. 'I'he control sensitivity will then fewer parts and 1523955 expensive t0 manufactura be Substantially uniform over the band. This Regarding the igt/@mm1 generano an impor.. condition is shown by the curve III of Fig. l-l. tant feature is trip provision of the resonant Taking a Specific example, if lt iS desired t0 Vary control reactance ientioned above. In the cir the Oscillator frequency over the necessary (1.0 cuits of Figs, 3, 4,25, 6 and '7, it w11] be noted 65 tov 2.0 m. c.) range for a superheterodyne receiver that the voltage iri1quadyature with the 0501i.. adapted to receive signals in the standard broadlator grid voltage Tis obtained by means of o, cast band, the control reactance should be tuned control reactance comprising a condenser C. to be resonant at, about 2.65 mesaoyoles. and 'I'his voltage will cause the control tube to ap.- under these circumstances, the sensitivity at the pear as a negative@ 'inductanee since the im- 70 ends of the band which represent the minimum pedance of the control vtube is proportional to sensitivity, will be 84% ofthe maximum sensitivthe negative reciprocal of the control reactance ity which will occur at about 11/2 megacycles. multiplied by the .mwtua1 conductance of the The preferred form of the control reactance is control tube and v[by the mutual conductance shown in Fig. 14 and constitutes a parallel resobetween the control grid and the output anode nant circuit X2 tuned below the frequency band and which over the operating range will then appear as a capacitive reactance. In the particular case above cited, the circuit should be tuned to be resonant at about .'75 megacycle and under these circumstances, the minimum control sensitivity within the band will be 84% of that at the extreme ends of the band, which represents the maximum sensitivity. The characteristic for this reactance is shown by curve IV in Fig. 11.

Y It will be understood that Figs. l2 to 14 are fragmentary views showing different forms of the control reactance which maybe used in the controlled oscillator.

The parallel resonant circuit has the advantage that the output anode to ground inter-electrode capacity of the oscillator tube is in shunt with that of the condenser in the resonant circuit and thus combines with the latter condenser without disturbing the desired properties of the circuit, whereas in the series circuit, it is effectively in shunt with the whole control reactance. The parallel inductance likewise provides an easier way of 'supplying energy to the anode, doing away with the resistor used in the series circuit.

Where the resonant circuit of Fig. 13 or Fig. 14 is used with the single control tube of Fig. 3 or Fig. 4 and where it is desired to provide phase correction by means of a resistive component in the control reactance it will, in general, be necessary to provide a plurality of resistances particularly if the oscillator frequency is to be varied over a wide range. As will be apparent, the undesired in-phase signal will depend upon the ratio of the reactive part of the control reactance to the impedance of the grld--to-plate inter-electrode capacitance of the control tube. It is then necessary to provide a control reactance whose conductance varies with frequency substantially similarly to the variations with frequency of this ratio. In general, satisfactory phase correction may be obtained by the use of two properly proportioned resistors one in shunt and one in series in the resonant control reactance. It will be apparent that more complicated networks may be used to obtain the desired quadrature and inphase signals.

Referring again to Fig. 1, the tuning means o the R. F. amplifier, 1st detector and oscillator, for example, the main tuning condenser in each unit, may be ganged and automatically and remotely controlled by a suitable tuning mechanism. as indicated by the broken-line representation. As such devices are well known in the art, further description of them is unnecessary except to point out that in general such devices include a switch for rendering the loutput circuit of the receiver inoperative while the tuning of the receiver takes place. When such devices are used with a receiver employing automatic irequency control, it is desirable that the A.VF..C. bias be .reduced to zero at least momentarily after the positioning of the tuning means is accomplished so that when the A. F. C. again becomes operative, it will lock in on the intended station and not on a neighboring channel. An important feature of the invention is the provision of electrical means by which this may be accomplished using the simple mechanical structure of the prior art. As will be apparent, the important point is to prevent the A. F. C. bias from building up until after the receiver is cornpletely tuned and the tuning condenser movement has ceased, whereas the switch for rendering the receiver inoperative is usually so arranged that it re-establishes transmission as the.

tuning operation is being completed, but before the tuning condenser has iinally come to rest. Following the practice oi' my invention, the resistance-capacitance lters -3 and t of Fig. 2 are arranged to have a long time-constant and a switch Si mechanically operated by the tuning means, as indicated in Fig. 2.' is provided by which the condensers of the filter may be shunted and thus discharged. When the switch S is closed, the voltage across the filter condensers and thus the A. F. C. bias is reduced to zero. When the switch S is opened, there will be no voltage across the condensers and thus electrically the A. F. C. bias will remain zero until sufcient time has elapsed to permit a charge to build u`p on the condensers. between the time the switch S opens and the time when the A. F. C. bias becomes operative will depend upon the time constant of the filter condensers and may be several seconds. Preferably,it should be sufliciently great to permit the tuning condenser to come completely to rest. Thus when the A. F. C. circuit locks in on the nearest station, it will be the desired station and not one on an adjacent channel. If desired, the time constant of the filters may be further augmented by the use of an additional condenser Cf in shunt therewith. The switch Si may be similar to the prior art switches used in such tuning devices to render the receiver output inoperative. A preferred form of such an automatic tuning means the switch is shown in the copending application of Leo B. Glaser et al., Serial No. 94,674, filed. August 6, 1936.

For purposes of this specification and claims, quadrature is dened as the phase relation between two vectors which are substantially perpendicular to each other, and thus the phase relation between two signals differing in phase by approximately to 270. Direct phase relation is deilned as the phase relations between` two vectors which are substantially co-linear with each other and thus the phase relation between two signals diiering in phase by approximately 0 or 180.

It will be seen from the illustrated embodiments and the above description that in each case the frequency-controlled unit comprises a loop circuit which includes the oscillator circuit, a thermionic device for transferring the signal in the loop circuit, means in the loop circuit for forming the quadrature voltage, and means for applying such voltage to the tank circuit. While the illustrated embodiments represent the most practical methods using commercially available tubes, the invention contemplates broadly the use of a loop circuit oi the character above mentioned embodying suitable means therein for providing the desired quadrature voltage.

While certain forms of the invention have been illustrated and described for the purpose of disclosure, it will be understood, of course, that the invention is not thus limited but is capable of various modifications other than those illustrated. Any such modifications 'are deemed to be within the scope of the invention.

Iclaim:

1. In an electrical system for forming frequency-controlled oscillations, means for generating self-sustained oscillations comprising a space discharge device and an associated resonant circuit including reactive elements, a control space discharge device having an input circuit and an output circuit, means for coupling said output circuit in operative relation with at The time interval quadrature relation quency-controlled desired resistive component.

2. In an electrical system for forming frequency-controlled oscillations. means for generating self-sustained oscillations comprising Aa space discharge device and an associated resonant circuit including reactive elements tunable over a band of frequencies, a control space discharge device having an input circuit and an output circuit,` means for coupling said output circuit in operative relation with at least a portion of a reactance of said resonant circuit, an impedance including a resistance, an inductive reactance and a capacitive reactance, said reactances being resonant at a frequency in the middle of said band, said impedance being energized byv a signal derived from said resonant circuit, and means for obtaining a signal substantially in phase quadrature relation with the signal across said output circuit from one of said reactances and for applying said obtained signal to said input' circuit, said resistance and said reactances. being so related as to reduce the transfer of a direct phase signal to said input circuit.

3. In an electrical system for forming frequency-controlled oscillations, means for generatingself-sustained oscillations comprising a space discharge device and an associated resonant circuit including reactive elements, a controlspace discharge device having an input circuit and an output circuit,` means for coupling said output circuit in operative relation withat least a portion of a reactance of' said resonant ciressaies obtaining a voltage substantially in phase opposition to said undesired voltage and for applying said last-named voltage to said input circuit.

5,'In an electrical systemfor forming frequency-controlled oscillations, means'for generating self-sustained oscillations comprising a space discharge device and an associated resonant circuit including reactive elements, a control space discharge device having an-input circuit and an output circuit, means for coupling said output circuit in operative relation with at least a portion oi a reactance of said resonant circuit, means for obtaining a signal voltage in phase quadrature relation with the voltage across said output circuit and for applying said quadrature voltage to said input circuit, there being inherent capacitive coupling between said input and output circuits whereby an undesired voltage in direct phase relation with said output circuit volt age is applied to said input circuit, a second control space discharge device coupled to said resonant circuit and having input and output circuits with inherent capacitive coupling therebetween, and means utilizing the inherent capacitive coupling between the input and output circuits of said second control space discharge device, ior obtaining a voltage substantially in phase opposition to said undesired voltage and for applying said last-named voltage tothe input circuit of said first control space discharge device.

v6. In an electrical system foriorming frequency-controiled oscillations, means for generating self-sustained oscillations comprising a space discharge device onant circuit including reactive elements, a con trol space discharge device having an'input cir. cuit and an output circuit, means for coupling said output circuit in operative relation with at least-.a portion of a. reactance of said resonant t circuit, means for obtaining a signal voltage in cuit, means for obtaining a signal voltage in phase with the voltage across said output circuit and for applying said quadrature voltage to said input circuit, there being inherent capacitive coupling put circuits whereby an undesired voltage in di- 'rect phase relation withsaid output circuit voltage is applied to said input circuit, and means in cluding a capacitive element for obtaining a volt-J in phase opposition to said v and for applying said last@ named voltage to saidv input circuit.

4. In an electrical system for forming ireoscillations, means for gener.. ating self-sustained oscillations comprising a space discharge device and an associated resonant circuit including reactive elements, a control between said input and out space discharge device having an input circuit and an output circuit, connections between said output circuit and said resonant circuit for connecting said output circuit in operative relation with at least a portion of a reactance of said A means Vfor obtaining a signal quadrature relation with the voltageacross said output circuit and for applying said quadrature voltage to said input circuit, there being inherent capacitive coupling between said input `said output circuit in phase quadrature relation-with the voltage across said output circuit and ture voltage to said input circuit, there being inherent capacitive coupling between said input and output circuits whereby an undesired voltage in direct phase relationwtih said output circuit voltagelis applied to said input circuit, a second control space discharge device coupled to said resonant circuit and havingA input and output circuits withinherent capacitive coupling therebetween, and means utilizing the inherent capacidevice, for obtaining a voltage substantially in phase opposition to said undesired voltage and for applying said last-named voltage to the input circuit of said rst control spacedis'charge device, theinput and output circuits of said second control device being coupledrespectively to the input and output circuits of said rst controlv trol space discharge device having an input circuitand an output circuit, means for coupling operative relation with at least a portion of a reactance of said resonant circuit, means for obtaining a signal voltage in and an associated rest forapplying said quadra-` accuses phase quadrature relation with the voltage across said output circuit and for applying said quadrature voltage to said input circuit, there being inherent capacitive coupling between said input and output circuits whereby an undesired voltage in direct phase relation with said output circuit voltage is applied to said input circuit, a second control space discharge device coupled to said resonant4 circuit andl having input and output circuits with inherent capacitive coupling therebetween, and means utilizing the inherent capacitive coupling between the input and output circuits of said second control space discharge device; i'or obtaining a voltage substantially in phase opposition to saidv undesired voltage and for applying said last-named voltage to the input circuit of said rst control space discharge device, the output circuit of said second control device being coupled to said resonant circuit in phase opposition to the output circuit of said rst com trol device.

8. In an. electrical system for forming frequency-controlled oscillations, means for generating self-sustained oscillations comprising aV said output circuit and said resonant circuit for connecting said `output circuit in operative relation with at least a portion of a reactance of said y means for applying said oscillations to said detector.

10. In a superheterodyne radio receiver including a first detector and an intermediate frequency amplifier, a frequency sensitive circuit energized by said amplier and adapted to form differential control voltages in accordance with the mistuning of said receiver, a pair of control space discharge devices each having an input circuit and an output circuit, means for applying said control voltages respectively to said input circuits, a third spacel discharge device and aresonant circuit associated therewith for' generating self-sustained oscillations, means for connecting one of said output circuits in operative relation with at least a portion of a reactance of said resonant circuit, means for obtaining a signal voltage inphase quadrature relation with the voltage acrosssaid one output circuit and for applying said quadrature voltage to the associated input circuit, means connecting said input circuits and said output Vcircuits in coopera.- tive relation such that the'inherent capacitive couplings of said control devices oppose one another, and means for applying said oscillations to said detector.

11. In an electrical system for forming frequency-controlled oscillations, a discriminator comprising a resonant circuit, means for supplying signal energy to said circuit, asecond resonant circuit reactively coupled to. said rst resonant resonant circuit, means for obtaining a signal .i

voltage in phase quadrature relation with the voltage across said output circuit and for applying said quadrature voltage to said input circuit,

there being inherent capacitive coupling between said input and output circuits whereby an undesired voltage in direct phase relation with said output circuit voltage is applied to saidinput circuit, and means including a second control space discharge devicel having an input circuit and .an output circuit with inherent capacitive couplingtherebetween, said last-named output circuit being connected in phase opposition to said rst-named output circuit, for obtaining a voltage substantially in phase opposition to said undesired voltage and for applying said lastnamed voltage to the input circuit of said rst control discharge device.

9.` In a superheterodyne radio receiver including a first detector and an intermediate Irequency amplifier, means connected to said amplifier for forming a control voltage whose amplitude varies in accordance with mistuning of said .herent capacitive coupling between said input and output circuits whereby an undesired voltage in direct phase relation with said output circuit voltage is applied to said input circuit, means for obtaining a voltage substantially in phase opposition to said undesired circuit voltage and for applying said last-named voltage to said input circuit for opposing'said undesired voltage, and

circuit, said second resonant circuit including an inductive element, a pair oi' diodes each having an anode and a cathode, a connection between each of said cathodes and said inductive element, a pair of serially connected resistances connected to said anodes ofsaid diodes, a connection conductive to unidirectional currents between the Amidpoint of said inductive velement and the common connection of said resistances, means for applying the voltage across said rst resonant circuit between said diode anodes and said midpoint of said inductive element, a second pair'of serially connected resistances connected to said diode anodes, and means for obtaining differential control signals from each of said second pair oi resistances. i l

12. In an electrical system -for forming frequency controlled oscillations, a resonant circuit including inductive and capacitive reactances, a space discharge device, connections between said device and said resonant circuit for adaptingv said device and said circuit to generate selfsustained oscillations, a control space discharge device having an input circuit and an output circuit, connections for coupling the output circuit to the resonant circuit, means for deriving a current in phase with the voltage across the resonantcircuit, a control reactance, connections for applying the said current to the control reactance to i'orm a voltage substantially in phase quadrature relation with the voltage across the resonant circuit, connections for supplying the quadrature voltage to the input circuit to form in the output circuit an edective reactance having an undesired resistive component, the control space discharge device, the current deriving means, the control reactance, and the connections for both the input and output circuits of the' control tube forming a closed loop circuit, and a second closed loop circuit for minimizing said undesired resistive component, comprising means for deriving a voltage in direct phase relation with the voltage across the resonant circuit and means for supplying such voltage to the input circuit, to form-in the output circuit a resistive component of opposite polarity with respect to said first mentioned component.

13. In combination with a resonant' tank circuit, a frequency control network comprising a pair of space discharge devices each including at least a cathode, a plate and a grid, an output circuit for one of said devices coupled to said tank circuit, another output circuit for the other of said devices coupled to said tank circuit in opposed relation to said first output circuit, a reactive pathA operatively associated with said tank circuit to derive alternating current voltage therefrom, means for impressing said voltage on each of the grids of said devices, and means for differentially biasing said grids in opposite polarity sense thereby to regulate the alternating current iiow in said output circuits.

14. In combination with an oscillator provided with a tunable tank circuit, a frequency control tube having an output circuit which is coupled to said tank circuit, said control tube including at least two control grids, a reactive path operatively associated with said tank circuit to derive alternating voltage therefrom, means for impressing said voltage on said grids, and means for impressing direct current voltages upon said grids in opposite polarity sense thereby to regulate the alternating current flow in said control tube output circuit.

15. In combination with a resonant circuit tuned to a desired frequency, a frequency variation network'including a pair of space discharge devices, each having an output electrode and space current controlling means, reactive means coupled to said resonant circuit for deriving therefrom alternating voltage substantially inV phase quadrature with the voltage across said resonant circuit, a source of varying voltage in response to which the frequency of said resonant circuit is to be varied, means for connecting said output electrodes to said resonant circuit to apply the output voltage of said devices to the resonant circuit, means for connecting said reactive means to the space current controlling devices to applying .the quadrature voltage thereto, and means for connecting said source to the means of said space current controlling means of said devices to apply said varying voltage thereto, the voltages applied by two of the aforesaid connecting means being in opposite phase, and the voltage applied by the other of said connecting means being in the same phase.

16. In combination with an oscillator provided With a resonant tank circuit, a frequency control network including a pair of space discharge devices, each having an output electrode and space current controlling means, reactive means coupled to said resonant circuit for deriving therefrom alternating voltage substantially in phase quadrature with the voltage across said resonant circuit, means responsive to variations of the oscillator frequency for producing control voltage, means for connecting said output electrodes to said resonant circuit to apply the output voltage of said devices to the resonant circuit, means for connecting saidV reactive means to the space current controlling means of said devices to apply the quadrature voltage thereto, and means for connecting said control voltage-producing means to the space current controlling means of said devices to apply the control voltage thereto, the voltages applied by two of the aforesaid connecting means being in opposite phase, and the voltage applied by the other of said connecting means being in the same phase.

1'7. In combination with a resonant circuit tuned to a desired frequency, a frequency variation network comprising a pair of space discharge devices each including an anode and space current controlling means, an output circuit for said devices coupled to said resonant circuit, said anodes being connected to a common point of said output circuit, reactive means coupled to said resonant circuit to derive alternating voltage therefrom, means for applying said voltage to the space current controlling means of said devices in opposite phase relation to control the space current therein, and means for applying diierentially-varying potentials to the space current controlling means of said devices to further control the space current therein.

CHARLES TRAVis. 

